Airbricks are at the base of external walls, normally below the
damp-proof course. In Victorian and Edwardian houses they were made
either from terracotta, glazed
or unglazed, or from cast iron.
In some houses, the airbricks are above the damp-proof course,
with a channel through the wall down to below the floor boards.
An example is shown below:
airbrick, originally glazed (1907)